1. Field
The present invention relates generally to data communication, and more particularly to a non-parametric matched filter receiver for use in wireless communication systems.
2. Background
Wireless communication systems are widely deployed to provide various types of communication such as voice, packet data, and so on. These systems may be multiple-access systems capable of supporting communication with multiple users and may be based on code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA), or some other multiple access techniques. These systems may also be wireless local area network (LAN) systems, such as those that conform to the IEEE standard 802.11b.
A receiver in a CDMA system typically employs a rake receiver to process a modulated signal that has been transmitted over a wireless communication channel. The rake receiver normally includes a searcher element and a number of demodulation elements, which are commonly referred to as “searcher” and “fingers,” respectively. Due to the relatively wide bandwidth of a CDMA waveform, the communication channel is assumed to be composed of a finite number of resolvable multipath components. Each multipath component is characterized by a particular time delay and a particular complex gain. The searcher then searches for strong multipath components in the received signal, and fingers are assigned to the strongest multipath components found by the searcher. Each finger processes its assigned multipath component to provide symbol estimates for that multipath component. The symbol estimates from all assigned fingers are then combined to provide the final symbol estimates. The rake receiver can provide acceptable performance for CDMA systems operated at low signal-to-interference-and-noise ratios (SINRs).
The rake receiver has a number of shortcomings. First, the rake receiver can provide unsatisfactory performance under certain channel conditions. This results from the rake receiver's inability to accurately model certain types of channels and to handle multipath components with time delays separated by less than one chip period. Second, a complicated searcher is normally needed to search the received signal to find strong multipath components. Third, a complicated control unit is also normally needed to decide if multipath components are present in the received signal (i.e., if they are of sufficient strength), assign fingers to newly found multipath components, de-assign fingers from vanishing multipath components, and support the operation of the assigned fingers. Because of the high sensitivity needed to find weak multipath components and the need for a small false alarm rate (i.e., declaring a multipath component to exist when in fact it does not), the searcher and control unit are normally quite complex.
There is therefore a need in the art for a receiver structure that can ameliorate the shortcomings noted above for the rake receiver.